From the viewpoint of global environment protection and energy saving, active research has been being carried out in recent years on materials that can reflect or shield infrared rays (infrared ray cut-off materials). Applications of infrared ray cut-off materials to the windows of houses, buildings, vehicles and the like have been studied in attempts to suppress temperature rises in rooms and vehicles by shielding rays in the infrared region out of incoming light.
In such infrared ray cut-off materials, known as colorless and transparent ones are tin-doped indium oxide (ITO), antimony-doped tin oxide (ATO) and the like. These materials are transparent without absorbing rays in the visible region, but absorb rays in the infrared region. These materials, however, involve a problem in that the absorption property in the infrared region is insufficient unless the addition amount, coat weight or film thickness is increased. Moreover, there is another problem in that the absorption of rays in the near infrared region is not very strong. In addition, there is a resource-related restriction because of the use of a rare metal or metals, such as indium (In) and tin (Sn), as a constituent or constituents.
Given the tinting of windowpanes, vehicle windows or the like, on the other hand, it is common to tint them black from the viewpoint of suppressing temperature rises, and carbon black is used in general. Carbon black is black when applied as a deep color, but presents a yellowish dull or brown hue when applied in the form of a thin film. For adjusting this color hue, bluish tinting or the like is additionally applied to carbon black before use. As organic black pigments, on the other hand, there are perylene black, azomethineazo-based black, and the like. These organic black pigments, however, allow rays in the infrared region to transmit without absorption. Moreover, such organic black pigments are known to be inferior in light resistance to inorganic pigments.
It is also known to tint the windowpanes of houses and buildings and the windows of vehicles, for example, by coating a fine particulate pigment directly on glass or films or applying adhesive layers, which have been tinted with the fine particulate pigment, to glass or films while making use of the transparency that is one of the characteristics of the fine particulate pigment. Especially, to tint liquid crystal displays (LCD), plasma display panels (PDP), organic EL panels and the like, preferred is a transparent vivid bluish, neutral gray color that is different from black or bluishness available when tinted in deep color.
As mentioned above, carbon black which is most frequently used, however, presents a yellowish dull or brown hue when applied in the form of a thin film. Carbon black is, therefore, not considered to be one that presents a transparent vivid bluish, neutral gray color. Perylene black presents a reddish purple hue when applied in the form of a thin film, and therefore, is not considered to be one that presents a transparent vivid bluish, neutral gray color. Azomethineazo-based black is equipped with a preferred color hue, but does not absorb rays in the infrared region. It is also a current situation that even among inorganic pigments, practically no material is found to be excellent in light transmission properties, to present a transparent vivid bluish, neutral gray color, and also to be excellent in the absorption performance of infrared rays.
For a pigment that is to be used as described above, light resistance is required. Expectations are, therefore, growing for inorganic pigments which are excellent in light resistance compared with organic pigments. Known examples include copper(Cu)-manganese(Mn)-iron(Fe)-based, composite oxide black pigments, which are prepared by a wet process, have high tinting power, and have bluish hues (Patent Documents 1 to 3). Also known are composite oxides, which present black colors and have an absorption property in a wavelength region ranging from the visible region to the infrared region (Patent Documents 4 to 6). It is also known to use carbon black in optical films for displays with a view to imparting a neutral gray tone (Patent Documents 7 and 8).